1. Field of the Invention
The present invention relates to a fingerprint sensor, and more particularly to a capacitive fingerprint sensor.
2. Description of the Prior Art
The principle of capacitive fingerprint sensor is to detect a capacitance variation between ridges and valleys of human finger. Normal depth of the valley is around 20-35 μm with air filled which has dielectric constant of 1. The dielectric constant of human tissue is about 4-8. Therefore, minute capacitance variation can be detected by the capacitive fingerprint sensor.
Referring to FIG. 1 to illustrate a detection method of a capacitive fingerprint sensor based on charge sharing algorithm. The capacitive fingerprint sensor includes a plurality of sensing units SU arranged in an array. The ridge and valley of the fingerprint FP has a capacitor C3 when the fingerprint FP touches the capacitive fingerprint sensor. Packaging the capacitive fingerprint sensor will form a dielectric layer OL, such as an oxide layer or epoxy etc., which has a capacitor C2. The capacitor C2 and the capacitor C3 connected in series has a capacitance value Cs which can be calculated by the following formula:1/Cs=1/C2+1/C3,
wherein C2 and C3 respectively represents the capacitance value of the capacitor C2 and the capacitor C3.
Measuring variation of the capacitance value Cs can be implemented by the following steps. First, the switch S1 is turned on and the switch S2 is turned off to precharge the capacitor C0 to bias voltage Va. Then the switch S1 is turned off and the switch S2 is turned on so that the charge on the capacitor C0 will redistribute to give a bias voltage V1 which can be calculated by the following formula:V1=Va*C0/(C0+C1+Cs),
wherein C0 and C1 respectively represents the capacitance value of the capacitor C0 and the capacitor C1. The capacitor C1 is parasitic stray capacitance of a circuit. The bias voltage V1 of sensing node of the sensing unit SU11 is buffered by buffer amplifier BA, and its output is bias voltage V2 which can be calculated by the following formula:V2=g*V1
wherein g is the gain of the buffer amplifier BA. Each sensing unit SU has one buffer amplifier BA, and the output of the buffer amplifier BA is multiplexed by row switch Sr1 and column switch Sc1-Sc3 before feed to sampling capacitor Csh. For example, the output of the sensing unit SU11 is controlled by the row switch Sr1 and column switch Sc1; the output of the sensing unit SU12 is controlled by the row switch Sr1 and column switch Sc2; and the output of the sensing unit SU13 is controlled by the row switch Sr1 and column switch Sc3. Finally, the analog to digital converter ADC converts the bias voltage V3 of the sampling capacitor Csh to a digital format. The buffer amplifier has gain variation from sensing unit to sensing unit caused by the process or other factors. In addition, due to the impact of the parasitic stray capacitance C1 and the capacitor C2, the uniformity of the bias voltage outputted from each sensing unit is poorer which impacts subsequent signal processing, such as direct current (DC) subtraction.
Besides, referring to FIG. 2 which shows a cross-sectional view of conventional package of the capacitive fingerprint sensor. A capacitive sensor chip 120 is attached to a substrate 110. Gold wires 130 are used to make connection between bonding pads 121 of the capacitive fingerprint chip 120 and a lead frame 111 on substrate 110. After chip attachment and wire bonding, the whole chip is molded with a high dielectric filler 140. However, during molding process, the difference of thermal expansion coefficient between the capacitive fingerprint chip 120 and the filler 140 will cause die mark or, as the height difference H shown in FIG. 2. Therefore, if the capacitive fingerprint sensor with warpage is disposed under a protective glass, an air layer or an adhesive gel is formed between the capacitive fingerprint sensor and the protective glass, which decreases the strength of the sensing signal and deteriorates the uniformity between the sensing units.
Accordingly, it is highly desirable to make the sensing signals outputted from each sensing unit have better uniformity.